examples.versioned_rows.versioned_rows — SQLAlchemy 2.0 Documentation

Release: 2.0.50 | Release Date: May 24, 2026

# SQLAlchemy 2.0 Documentation

# Source code for examples.versioned_rows.versioned_rows

    """Illustrates a method to intercept changes on objects, turning
    an UPDATE statement on a single row into an INSERT statement, so that a new
    row is inserted with the new data, keeping the old row intact.

    """

    from sqlalchemy import Column
    from sqlalchemy import create_engine
    from sqlalchemy import event
    from sqlalchemy import ForeignKey
    from sqlalchemy import Integer
    from sqlalchemy import String
    from sqlalchemy.ext.declarative import declarative_base
    from sqlalchemy.orm import attributes
    from sqlalchemy.orm import backref
    from sqlalchemy.orm import make_transient
    from sqlalchemy.orm import relationship
    from sqlalchemy.orm import Session
    from sqlalchemy.orm import sessionmaker

    class Versioned:
        def new_version(self, session):
            # make us transient (removes persistent
            # identity).
            make_transient(self)

            # set 'id' to None.
            # a new PK will be generated on INSERT.
            self.id = None

    @event.listens_for(Session, "before_flush")
    def before_flush(session, flush_context, instances):
        for instance in session.dirty:
            if not isinstance(instance, Versioned):
                continue
            if not session.is_modified(instance):
                continue

            if not attributes.instance_state(instance).has_identity:
                continue

            # make it transient
            instance.new_version(session)
            # re-add
            session.add(instance)

    Base = declarative_base()

    engine = create_engine("sqlite://", echo=True)

    Session = sessionmaker(engine)

    # example 1, simple versioning

    class Example(Versioned, Base):
        __tablename__ = "example"
        id = Column(Integer, primary_key=True)
        data = Column(String)

    Base.metadata.create_all(engine)

    session = Session()
    e1 = Example(data="e1")
    session.add(e1)
    session.commit()

    e1.data = "e2"
    session.commit()

    assert session.query(Example.id, Example.data).order_by(Example.id).all() == (
        [(1, "e1"), (2, "e2")]
    )

    # example 2, versioning with a parent

    class Parent(Base):
        __tablename__ = "parent"
        id = Column(Integer, primary_key=True)
        child_id = Column(Integer, ForeignKey("child.id"))
        child = relationship("Child", backref=backref("parent", uselist=False))

    class Child(Versioned, Base):
        __tablename__ = "child"

        id = Column(Integer, primary_key=True)
        data = Column(String)

        def new_version(self, session):
            # expire parent's reference to us
            session.expire(self.parent, ["child"])

            # create new version
            Versioned.new_version(self, session)

            # re-add ourselves to the parent
            self.parent.child = self

    Base.metadata.create_all(engine)

    session = Session()

    p1 = Parent(child=Child(data="c1"))
    session.add(p1)
    session.commit()

    p1.child.data = "c2"
    session.commit()

    assert p1.child_id == 2
    assert session.query(Child.id, Child.data).order_by(Child.id).all() == (
        [(1, "c1"), (2, "c2")]
    )